It may be transplant shock, or if your plants flowered within the past 5 years, it could be that they are in the process of dying. Once these plants flower, it is their internal signal to become compost for their offspring. If your plants haven't flowered, then I'd chalk it up to transplant shock plus too much water.

Fishtail Palms (Caryota mitis) get their name from the shape of the leaflets, which are about 6" long and 4" wide. A mature plant will grow 6'-8' tall. Palms do best in partial shade and require excellent drainage as they detest having their roots sitting in water. During the winter keep the soil only slightly moist. Water more liberally in spring and summer when the plant is actively growing, allowing the top of the soil to just barely dry out between waterings. Yellowing leaves indicate overwatering, so water less frequently than you have been. Red spider mites, scale insects and mealybugs can all attack palms. These are sucking insects and can leave a sticky liquid behind as they feed. Carefully inspect the undersides of the leaves for insect presence. If you find any, hose them off with a strong stream of water. If your plant is growing in a pot indoors, you can take it outdoors to a shady area to hose it off, or put it in the shower, to clean the leaves off. Use tepid, not hot water in the shower! Palms are not heavy feeders, but you can use a diluted half-strength liquid fertilizer every 4 weeks in place of a watering, during the spring and summer months.